Flash vacuum pyrolysis (FVP) of 1-(5-(13)C-5-tetrazolyl)isoquinoline 18 generates 1-((13)C-diazomethyl)isoquinoline 19 and 1-isoquinolyl-((13)C-carbene) 22, which undergoes carbene-nitrene rearrangement to 2-naphthylnitrene 23. The thermally generated nitrene 23 is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes 26 and 27 under the FVP conditions. The (13)C label distribution in the cyanoindenes was determined by (13)C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo[c]cyclohepta-1,2,4,6-tetraene 32 bifurcating to 2-naphthylnitrene 23 and 2-aza-benzobicyclo[3.2.0]heptatriene 39 (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo[d]bicyclo[4.1.0]hepta-2,4,6-triene 34 and 3-aza-benzo[d]cyclohepta-2,3,5,7-tetraene 35. Product studies demand that the major path is route a1 via 2-naphthylnitrene 23, which then undergoes direct ring contraction to 1-cyanoindene; but the (13)C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene 22 is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)isoquinoline 28 similarly generates 3-diazomethylisoquinoline 29 and 3-isoquinolylcarbene 30, which rearranges to 3- and 2-cyanoindenes 26 and 27.

A set of chemical reactions is postulated to account for the formation of the macrocyclic porphin structure, basic to the pyrrole derivatives chlorophyll, protoporphyrin, heme and bilirubin, important in photosynthesis, respiration and digestion. A set of equations is given for the prebiotic synthesis of porphin derivatives from the simple molecules; cyanoacetylene, diacetylene, carbon monoxide and ammonia that have been detected in space. A number of isomers of hydrogenated porphin arise which may lose hydrogen to give ultimately porphin and its dehydrogenated derivative. The reactions, while not unique, provide a pathway which has been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the HF and MP2/6-31G* level.

The relationship between 4-quinolylcarbene 17, 3-quinolylcarbene 21, 2-quinolylcarbene 25, and 1-naphthylnitrene 35 has been explored experimentally and computationally. The diazomethylquinolines generated from (5-tetrazolyl)quinolines or 1,2,3-triazolo[1,5-a]quinoline by conventional flash vacuum pyrolysis (FVP) were observed by IR spectroscopy. The carbenes were generated by falling solid flash vacuum pyrolysis (FS-FVP). 4-Quinolylcarbene 17 was found to rearrange to 3-quinolylcarbene 21 and then to 2-quinolylcarbene 25, and finally via 1-naphthylnitrene 35 to 1-cyanoindene 36, which then isomerizes to 3- and 2-cyanoindenes 12 and 13. The thermal rearrangement of 2-quinolylcarbene to 1-naphthylnitrene was verified by ESR spectroscopy. The reaction mechanism has been elucidated with the help of calculations of the structures and energies of the quinolylcarbenes and 1-naphthylnitrene and the intervening aza-benzobicyclo[4.1.0]heptatrienes, aza-benzocycloheptatetraenes, and aza-benzocycloheptatrienylidenes and the transition states connecting them at the B3LYP/6-31G* level. The nonobserved 1,2-hydrogen shifts in aza-benzocycloheptatetraenes/aza-benzocycloheptatrienylidenes are found to have very high activation barriers.

Thermodynamic constants of the ionisation of the acid imino-group of uridine 5'-monophosphate (UMP-5') and polyuridylic acid (poly-U) have been evaluated a t various ionic strengths and temperatures by potentiometry. The low temperature titration of poly-U is consistent with the secondary structure probably being a double-helix.

The Raman spectra have been obtained of poly(adenylic acid) in the solid state and aqueous solutions at various pH values. For comparison purposes, the spectra were run of low molecular weight analogs, including adenosine, 2'-deoxyadenosines, adenosine, 5'-monophosphate, and adenosine 3'-monophosphate. The differences in the spectra of the single-and double-stranded helices of poly(adenylic acid) are interpreted in terms of the protonation of the species, hydrogen bonding, and base stacking. he chemical structure of the nucleic acids dictates that the factors responsible for the secondary structure and the mechanism of their enzymatic replication must reside in the nitrogenous bases of the polynucleotide chains. These bases stack and base pair through hydrogen bonds to produce specific secondary structures. Methods of characterizing the secondary structures and the nature of the specific interactions of nucleic acids in aqueous solution are limited. It is the purpose of this paper to report the use of laserexcited Raman spectroscopy to reveal some aspects * To whom correspondence should be addressed.(1) G.